RNA's central role in biological function and regulation has become increasingly apparent. A vast number of non-coding RNAs have been discovered, and there is extensive cellular regulation by RNAs such as riboswitches and microRNAs, by alternative splicing of pre-mRNAs, by association of RNA binding proteins with families of mRNAs encoding functionally related proteins, and, likely, by the involvement of ncRNAs in epigenetics. The importance of RNA machines in cellular function and regulation is also profound, with roles in telomere maintenance, pre-mRNA splicing, protein synthesis, and membrane targeting. And researchers are increasingly turning to RNA for cellular engineering and as a therapeutic target. In-depth studies of RNAs that allow dissection of functional properties and uncovering of fundamental RNA properties are more important than ever. Over the past two decades, group I introns have served as unparalleled systems for developing broad experimental approaches for RNA study and for deep investigation of RNA structure and function. Comparing and contrasting the properties of RNA and protein enzymes has allowed generalization of fundamental features of biological catalysts and has, conversely, highlighted critical functional distinctions between these structured, biological macromolecules. Over the last funding period single molecule methods, new structure-function tools, and new methods to probe RNA dynamics have emerged, and our group has made foundational contributions to these breakthroughs and has exploited their power. Also emerging were the first x-ray structures of group I introns, opening a new era of RNA structure-function studies. Our proposed studies capitalize on these highly developed tools, the recent structural information, and the extremely tractable group I RNA system to ask RNA structure-function questions that lie at the frontier of our understanding and to answer these questions at unprecedented depth. We will continue to pioneer approaches that will be broadly applicable to other, more complex RNA and RNA/protein systems. We will dissect the transition states for P1 helix docking to the Tetrahymena and Azoarcus ribozyme cores, while developing innovative experimental/computational approaches to dissect conformational transition paths. We will use comparative structure-function analyses to understand how the Azoarcus group I ribozyme is able to utilize 6 kcal/mol more tertiary stabilization energy than the Tetrahymena ribozyme. We will incisively probe competing mechanistic proposals for group I ribozyme active site interactions, unifying information from structural and functional studies. Finally, we will take on the grand challenge of understanding how a functional RNA is assembled, in particular probing the roles and mode of action of remote elements that facilitate function within the catalytic core.